[Gap Decorations]: Remove redundant serialization tests

This CL removes the redundant serialization test for the Gap Decorations
feature. Serialization of these properties are already tested via the
`test_valid_value` helper method [1] which these properties already call
for testing valid values.

[1]:
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/external/wpt/css/support/parsing-testcommon.js;l=9?q=test_valid_value&ss=chromium%2Fchromium%2Fsrc

Bug: 357648037
Change-Id: Ic5367b225c606a2de7e9abb4cd6b916640e6c34d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6534250
Reviewed-by: Javier Contreras <[email protected]>
Commit-Queue: Sam Davis Omekara <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1459617}

Author: Sam Davis Omekara